Suprise success: Enda Walsh (right) and John Tiffany thought Once was ‘undoable’ as a musical (Picture: Daniel Lynch)

What’s the secret to making a good musical? Make one you think will never work. That was the policy playwright Enda Walsh and director John Tiffany pursued when they set out to adapt John Carney’s 2006 Irish indie movie, Once – and it’s turned out pretty well. Once picked up eight Tony awards last year and hits the West End next week.

‘When I first watched the film, I thought, this is undoable as a musical: it’s a two-hander and nothing happens,’ says Walsh, his bewilderment still palpable. ‘But the emotion of the piece just builds on you. I still don’t understand what happens in the middle of the second half of our version, to be honest.’

Once has crept into the West End almost as quietly as it stole into New York. It’s not a musical that ‘flies its knickers on its sleeve’, as Tiffany puts it: its New York success depended on word of mouth. It’s a musical with no big numbers, chorus line, orchestra, stars and not much of a plot; an anti-musical, almost.

Set to a folky acoustic soundtrack originally penned by Irish band The Frames, it’s the classic story of guy meets girl, except in this instance guy, a busker, is nursing a broken heart and is determined never again to pick up his guitar. Girl, who is Czech, persuades him to start singing. There’s a bonding moment over a vacuum cleaner and, well, you’ll just have to find out how it ends. And get there early – there’s a jamming session in the on-stage bar before the show starts. You don’t get that in The Book Of Mormon.

‘It’s a love story,’ says Walsh, who has a habit of spurting out several expletive-laden sentences in a mass of verbal spaghetti. ‘And the reason why it works is that there will be 1,000 people in the audience and they’ll all have their own love stories, and all these love stories will be in the air. After my grandma died, my grandad said to me: “It must have been 1914; I was walking through Dublin, down O’Connell Street, I looked around and there was a girl. And I’ve thought about that girl every week.” I couldn’t believe it: I thought – what about grandma? But there’s something in the idea that a single life can carry many love stories.’

Walsh and Tiffany, who met via the National Theatre Of Scotland, look more like the sort of people who would prefer to lose a night on the beer than wine and dine Broadway producers. Yet both have extraordinary pedigrees.

Dublin-born Walsh is the writer of Steve McQueen’s award-winning IRA film, Hunger, and the incendiary playwright behind Disco Pigs, which he later turned into a film starring Cillian Murphy. Tiffany directed Black Watch – the global smash hit theatre show about the Scottish regiment serving in Iraq. Neither had any experience of Broadway or musicals but both innately understand the simple magic of an acoustic tune – which is precisely the power that Once depends on.

‘It’s a very Irish thing, of course, but it’s also a very Yorkshire thing,’ says Tiffany, from Huddersfield. ‘My dad was in a brass band and I grew up among working-class men who found it easier to communicate through music than words.’

‘We’re just pushing words into darkness,’ adds Walsh, obliquely. ‘Actually, there has always been music in my work. Once is a bit like Disco Pigs. And even Misterman [his 2011 stage psychodrama, again starring Murphy] had a s***-load of Doris Day in it. My dad was a furniture salesman who adored MGM musicals, so I grew up on that. I often think: “Why the f*** can’t people walk down the road and start singing?”’

Both get a kick out of what Tiffany calls the musical’s ‘lo-fi’ quality (even the costumes came from charity shops) and that something so ‘delicate’ has made such a loud noise in New York. Certainly, no one quite knew what to make of them at the Tony awards. Tiffany, for whom the highlight of the evening was meeting Angela Lansbury outside the gents, recounts a journalist shoving a microphone into his face with the words: “So, John, how did it feel when you got the part?” But Once has become its own fairy tale. ‘The whole thing has just been really honest,’ says Walsh. ‘The show is basically just like being sung to down the pub.’